Pipes to be installed in pipelines are commonly coated with a corrosion barrier layer, for example a thin resinous layer which is bonded to the pipe surface and, so long as it remains intact, serves as a water barrier to protect the pipe against corrosion, although in practice it is usual topprovide further protection against corrosion by a system of cathodic protection. In the installation of a pipeline both the pipe and the thin resinous layer are susceptible to mechanical damage caused by impact of rocks during lowering and backfilling, and denting by rocks due to the high static loading during hydrostatic testing operations. To prevent such damage there is a great need in the pipeline industry for a protective coating which will absorb impact and diffuse penetration loads likely to cause damage. Currently available materials which are used on a small scale for this purpose include particle board, which can withstand impact up to about 60 Joules, and fibre-reinforced cement, which can withstand impact up to about 20 Joules. However, these materials are not suitable for pipeline applications in rocky terrain where much greater mechanical protection is required.
One of the difficulties in making a suitable selection from the available materials is that the material must not only have the requisite impact and penetration resistance but must also have sufficient flexibility to accommodate bending of the pipe up to a certain amount typically 1.5.degree. per pipe diameter permanent deflection.
In order to avoid damage to the pipe, as well as the coating, from rocks, particularly where a pipe is to be buried in a rocky terrain, the common practice is to truck sand from other areas and to embed the pipe in the sand at a level elevated from the rocky base. This is a very costly procedure which can be obviated by the use of a suitably structured coating.
One such coating, as applied to a metal pipe, is described in U.S. Pat. No. 4,611,635, Harold F. Jarvis. The coating essentially consists of a water-permeable layer of impact-resistant and penetration-resistant cladding material structured in such a way as to accommodate limited bending of the pipe.
The cladding material may be concrete, but other materials having the requisite impact resistance, penetration resistance and permeability to water may also be used. The relevant criteria of such a material for use in pipeline applications are as follows:
______________________________________ Minimum Compressive Strength 17 MPa Maximum Elongation 2.2% Minimum Impact Resistance 450 Joules (measured with a 25 mm diameter tup with a 10 Kg weight) Minimum Penetration Resistance 2500 Kg. (measured with a 25 mm diameter blunt rod) ______________________________________
The volume resistivity of the material when saturated with water should be less than five times the volume resistivity of water, and freeze-thaw cycling should have no effect.
The coating of cladding material described in U.S. Pat. No. 4,611,635 is primarily intended for pipe of relatively small diameter, up to about 18 inches. However, with pipe of larger diameter such a coating in practice offers resistance to bending of the pipe and tends to act as a rigid jacket, the rigidity increasing with pipe diameter. The present invention overcomes this limitation by providing a coating of inherently rigid cladding material having a novel structural configuration which enables it to accommodate limited bending of the pipe to which it is applied, even though the pipe be of relatively large diameter.
Concrete-coated metal pipes are well known in the art. Canadian Pat. Nos. 959744 and 1076343 relate to methods and apparatus for applying concrete coatings to pipes. These coatings are usually applied to pipes intended for offshore installations where weight and negative buoyancy are the major considerations, but are generally unsuitable for inland installations owing to the rigidity of the product and therefore its inability to accommodate bending of the pipe as it is laid. The present invention provides a protective coating which does not have these limitations and which is especially suitable for inland pipeline installations.